Recently collected data indicate that naltrindole (NTI), a specific delta opioid receptor antagonist, has features of a useful medicine for treating people abusing cocaine. Although the results from preliminary research are very encouraging, those results are limited by the small numbers of subjects used, and the fact that only a limited range of acute doses of NTI were used. Given both the promise of the initial results and their limitations, the aims are straightforward. The aim is to assess the effects of NTI, thereby, laying a basis for the further development of pharmacological agents that safely modify cocaine's positive affective, positively reinforcing potential. More specifically, research is planned, with respect to cocaine's effects, to (a) observe the effects of a range of doses of NTI, (b) determine if NTI is orally effective, and (c) see if NTI's effects change with repeated daily dosing. Additionally, other experiments are planned, for example, to assess NTI's ability to modify responses conditioned to cocaine's effects. Also, NTI's effects will be assessed by testing it with respect to amphetamine's reinforcement. Because NTI may have limiting side-effects, other specific delta opioid receptor antagonists' ability to modify cocaine's effects will be examined.